Recuperators are commonly used to recover energy from flue gas. It has been known to provide inner and outer tubes in which the flow of cold air is through the inner tube into the inner closed end of the outer tube and thence outwardly in a passage of annular configuration to a hot air chamber or vice versa. The inner tube passes through the hot air chamber and the outer tube extends substantially across the flue gas passage so that the entering cold air is somewhat heated in the hot air passage and is substantially heated as it passes between the outer wall of the inner tube and the inner wall of the outer tube substantially across the hot flue gas passage.
In accordance with the present invention the inner tube, which is not subjected to temperatures approaching the flue gas temperatures, is made of steel, preferably a suitable stainless steel. The outer tube which of course is exposed directly to the extremely hot flue gases is made of a ceramic material.
The spacing between the outer wall of the inner tube and the inner wall of the outer tube is relatively small so that the cool air is efficiently heated as it traverses the inner surface of the hot outer ceramic tube.
In accordance with the present invention the outer ceramic tube has an annular support surface at one end and the necessary sealing of this tube with respect to the apertured surface which supports it, is provided simply by causing the weight of the ceramic tube to apply pressure to this annular support surface which is supported on a suitable heat-resistant fiber sealing pad.
Preferably the combination of the inner tube and outer tube extends upwardly from the lower surface of the flue gas passage, in which case the outer ceramic tube has an upper closed end and its lower open end provides the sealing action through the weight of the ceramic tube resting on the annular sealing pad. This permits the outer ceramic tube to be removed simply by lifting through an access opening provided at the top of the flue gas passage.
It has been found that the inner metallic tube does not attain excessive temperatures, even under conditions of partial failure of the system, and accordingly it has been found practical to provide the inner open ended metal tubes with a simple threaded connection at the lower ends thereof to a threaded fitting provided at an opening in the wall dividing the cold air manifold and the superimposed hot air chamber. Thus both the inner and outer tubes can be readily removed and replaced through an access opening provided in the upper wall of the flue passage.